Characteristics of an 18 MV photon beam from a Therac 20 Medical Linear Accelerator

Measurement and comparison of head scatter factor for 7 MV unflattened (FFF) and 6 MV flattened photon beam using indigenously designed columnar mini phantom

AIM

To measure and compare the head scatter factor for 7 MV unflattened and 6 MV flattened photon beam using a home-made designed mini phantom.

BACKGROUND

The head scatter factor (Sc) is one of the important parameters for MU calculation. There are multiple factors that influence the Sc values, like accelerator head, flattening filter, primary and secondary collimators.

MATERIALS AND METHODS

A columnar mini phantom was designed as recommended by AAPM Task Group 74 with high and low atomic number material for measurement of head scatter factors at 10 cm and d max dose water equivalent thickness.

RESULTS

The Sc values measured with high-Z are higher than the low-Z mini phantoms observed for both 6MV-FB and 7MV-UFB photon energies. Sc values of 7MV-UFB photon beams were smaller than those of the 6MV-FB photon beams (0.6-2.2% (Primus), 0.2-1.4% (Artiste) and 0.6-3.7% (Clinac iX (2300CD))) for field sizes ranging from 10 cm × 10 cm to 40 cm × 40 cm. The SSD had no influence on head scatter for both flattened and unflattened beams. The presence of wedge filters influences the Sc values. The collimator exchange effects showed that the opening of the upper jaw increases Sc irrespective of FF and FFF.

CONCLUSIONS

There were significant differences in Sc values measured for 6MV-FB and unflattened 7MV-UFB photon beams over the range of field sizes from 10 cm × 10 cm to 40 cm × 04 cm. Different results were obtained for measurements performed with low-Z and high-Z mini phantoms.

Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom

To measure and compare the head scatter factor for flattened (FB) and unflattened (FFF) of 6MV and 10MV photon beam using indigenously designed mini phantom. A columnar mini phantom was designed as recommended by AAPM Task Group 74 with low and high atomic number materials at 10 cm (mini phantom) and at approximately twice the depth of maximum dose water equivalent thickness (brass build-up cap). Scatter in the accelerator (Sc) values of 6MV-FFF photon beams are lesser than that of the 6MV-FB photon beams (0.66-2.8%; Clinac iX, 2300CD) and (0.47-1.74%; True beam) for field sizes ranging from 10 × 10 cm² to 40 × 40 cm². Sc values of 10MV-FFF photon beams are lesser (0.61-2.19%; True beam) than that of the 10MV-FB photons beams for field sizes ranging from 10 × 10 cm² to 40 × 40 cm². The SSD had no influence on head scatter for both flattened and unflattened beams and irrespective of head design of the different linear accelerators. The presence of field shaping device influences the Sc values. The collimator exchange effect reveals that the opening of the upper jaw increases Sc irrespective of FB or FFF photon beams and different linear accelerators, and it is less significant in FFF beams. Sc values of 6MV-FB square field were in good agreement with that of AAPM, TG-74 published data for Varian (Clinac iX, 2300CD) accelerator. Our results confirm that the removal of flattening filter decreases in the head scatter factor compared to flattened beam. This could reduce the out-of-field dose in advanced treatment delivery techniques.

Study of head scatter factor in 4MV photon beam used in radiotherapy

The 4 MV photon beam offers equal build-up region behavior like Co-60 beam and it plays a major role in head and neck and pediatric radiotherapy. In this study an attempt is made to study the head scatter factor (SC) for 4 MV photon beam using locally designed PMMA and Brass miniphantoms. The SC is measured in combination of PMMA miniphantom with 0.6 cc chamber and Brass miniphantom with 0.6 cc and 0.13 cc chambers. The measured SC is compared with the literature data and it agrees within ± 1.98%. The study reveals that either 0.13 cc or 0.6 cc chamber with PMMA or Brass phantom materials can be used for SC measurements in a 4 MV photon beam. The variation of SSD does not alter the head scatter factor. The collimator exchange effect is found to be within 1, and it is less than that of other linear accelerators. It is also found that the presence of internal wedge has significant contribution to head scatter factor. The Phantom scatter factor is also calculated and it agrees within ±1% with published data.

Photon spectral characteristics of dissimilar 6 MV linear accelerators

This work measures and compares the energy spectra of four dosimetrically matched 6 MV beams, generated from four physically different linear accelerators. The goal of this work is twofold. First, this study determines whether the spectra of dosimetrically matched beams are measurably different. This study also demonstrates that the spectra of clinical photon beams can be measured as a part of the beam data collection process for input to a three-dimensional (3D) treatment planning system. The spectra of 6 MV beams that are dosimetrically matched for clinical use were studied to determine if the beam spectra are similarly matched. Each of the four accelerators examined had a standing waveguide, but with different physical designs. The four accelerators were two Varian 2100C/Ds (one 6 MV/18 MV waveguide and one 6 MV/10 MV waveguide), one Varian 600 C with a vertically mounted waveguide and no bending magnet, and one Siemens MD 6740 with a 6 MV/10 MV waveguide. All four accelerators had percent depth dose curves for the 6 MV beam that were matched within 1.3%. Beam spectra were determined from narrow beam transmission measurements through successive thicknesses of pure aluminum along the central axis of the accelerator, made with a graphite Farmer ion chamber with a Lucite buildup cap. An iterative nonlinear fit using a Marquardt algorithm was used to find each spectrum. Reconstructed spectra show that all four beams have similar energy distributions with only subtle differences, despite the differences in accelerator design. The measured spectra of different 6 MV beams are similar regardless of accelerator design. The measured spectra show excellent agreement with those found by the auto-modeling algorithm in a commercial 3D treatment planning system that uses a convolution dose calculation algorithm. Thus, beam spectra can be acquired in a clinical setting at the time of commissioning as a part of the routine beam data collection.

Response of CaSO4:Dy based TL dosemeter system to high-energy photon beams: theoretical simulation

Response of thermoluminescence (TL) discs under different filter regions of a CaSO(4):Dy based TL dosemeter system was simulated to high-energy photon beams in the energy range of 1.25 MeV to 24 MV ( approximately 9 MeV). This was done using FLUKA Monte Carlo code and also experimentally verified for some energy points. Response of disc D1 under metal filter combination was found to increase with photon energy, whereas that for the discs under polystyrene filter and open window regions of the dosemeter decreases continuously. The changes in the response of the discs under polystyrene filter and open window were attributed to the lack of build-up material. The increase in the response of disc D1 was due to the contribution from secondary electrons produced through Compton and pair production processes mainly arising out from the metal filter combination. The knowledge of the change in the response of individual discs and the ratio of discs' responses under different filter regions of the dosemeter system could be used for the measurement of energy of bremsstrahlung radiation that exists in and around high-energy electron accelerator and could be used for accurate evaluation of personal dose equivalent in high-energy photon field.

Measurement of back-scattered radiation from micro multileaf collimator into the beam monitor chamber from a dual energy linear accelerator

Measurements designed to find the collimator backscatter into the beam monitor chamber from Micro Multileaf collimator of 6 MV photon beams of the Siemens Primus linear accelerator were made with the help of dose rate feedback control. The photons and electrons backscattered from the upper and lower secondary collimator jaws give rise to a significant increase in the ion charge measured by monitor chamber. This increase varies between the different accelerators. The output measurements were carried out in air at the isocenter. The effect of collimator backscatter was investigated by measuring the pulse width, number of beam pulses per monitor unit, monitor unit rate and dose for different mMLC openings. These measurements were made with and without dose rate feedback control, i.e., with constant electron beam current in the accelerator. Monitor unit rate (MU/min) was almost constant for all field sizes. The maximum variation between the open and the closed feedback control circuits was 2.5%. There was no difference in pulse width and negligible difference in pulse frequency. Maximum value of backscattered radiation from the micro Multileaf collimator into the beam monitor chamber was found to be 0.5%.

Ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

A new method to measure the effect of the backscatter into the beam monitor chambers in linear accelerators is introduced from first principles. The technique, applicable to high-energy photon beams, is similar to the well-known telescopic method although here the heavy blocks are replaced by a very small, centred block on the shadow tray, thus the name 'ecliptic method'. This effect, caused mainly by backscattering from the secondary collimators, is known to be an output factor constituent and must be accounted for when detailed calculations involving the machine's head are required. Since its magnitude is generally small, experimental errors might obscure the behaviour of the phenomenon. Consequently, the procedure introduced goes along with an uncertainty assessment. Our theory was confirmed via measurements in cobalt-60 beams, where the studied effect does not contribute to the output factor. Measurements were also performed on our Saturne 41 linear accelerator and the results were qualitatively similar to those described elsewhere. The collimation systems were studied separately by varying one jaw setting while keeping the other at its maximum value. In the light of these results, we deduced an algorithm that can correlate the former data with the effect of backscattering to the beam monitor chambers for any rectangular field within 0.5%, which is of the order of the experimental uncertainty (0.6%). As we show, the experimental procedure is safe, simple, not invasive for the linac and requires only basic dosimetry equipment.

Measurement of head scatter factors of linear accelerators with columnar miniphantoms

The measurement of linear accelerator head scatter factors or in-air output factors, Sc, with columnar miniphantoms is refined in this work. Columnar miniphantoms are constructed from water equivalent materials: solid water and M3, and materials with higher mass density and atomic number: copper and lead. The change in the value of Sc from a 4-cm X 4-cm to a 40-cm X 40-cm field is different by 22% +/- 3%, 18% +/- 2%, and 10% +/- 3% for 6, 15, and 23 MV x rays, respectively, when measured with water equivalent or lead miniphantoms of 10 gm/cm2 depth. Based on measurements of transmission factors in solid-water miniphantoms of different depths, it is demonstrated that the beam energy spectra decreases in energy with increased field size. These changes in beam energy spectra alter the transmission and scatter of radiation and buildup of the dose in the miniphantom even if the miniphantom is made of water-equivalent material. These changes underlie the alteration in Sc when measured by miniphantoms fabricated from materials of different atomic number. It is shown that miniphantoms designed with a depth just adequate to stop contamination electrons will minimize these distortions due to transmission and scatter of radiation and buildup of dose in the miniphantom. Use of a miniphantom constructed from water-equivalent material with a depth appropriate for the x-ray energy being measured is the preferred method for determining Sc for dosimetry in water.

An investigation of accelerator head scatter and output factor in air

Our purpose in this study was to investigate whether the Monte Carlo simulation can accurately predict output factors in air. Secondary goals were to study the head scatter components and investigate the collimator exchange effect. The Monte Carlo code, BEAMnrc, was used in the study. Photon beams of 6 and 18 MV were from a Varian Clinac 2100EX accelerator and the measurements were performed using an ionization chamber in a mini-phantom. The Monte Carlo calculated in air output factors was within 1% of measured values. The simulation provided information of the origin and the magnitude of the collimator exchange effect. It was shown that the collimator backscatter to the beam monitor chamber played a significant role in the beam output factors. However the magnitude of the scattered dose contributions from the collimator at the isocenter is negligible. The maximum scattered dose contribution from the collimators was about 0.15% and 0.4% of the total dose at the isocenter for a 6 and 18 MV beam, respectively. The scattered dose contributions from the flattening filter at the isocenter were about 0.9-3% and 0.2-6% of the total dose for field sizes of 4x4 cm2-40x40 cm2 for the 6 and 18 MV beam, respectively. The study suggests that measurements of head scatter factors be done at large depth well beyond the depth of electron contamination. The insight information may have some implications for developing generalized empirical models to calculate the head scatter.

Using Monte Carlo simulations to commission photon beam output factors—a feasibility study

This study investigates the feasibility of using Monte Carlo methods to assist the commissioning of photon beam output factors from a medical accelerator. The Monte Carlo code, BEAMnrc, was used to model 6 MV and 18 MV photon beams from a Varian linear accelerator. When excellent agreements were obtained between the Monte Carlo simulated and measured dose distributions in a water phantom, the entire geometry including the accelerator head and the water phantom was simulated to calculate the relative output factors. Simulated output factors were compared with measured data, which consist of a typical commission dataset for the output factors. The measurements were done using an ionization chamber in a water phantom at a depth of 10 cm with a source-detector distance of 100 cm. Square fields and rectangular fields with widths and lengths ranging from 4 cm to 40 cm were studied. The result shows a very good agreement (< 1.5%) between the Monte Carlo calculated and the measured relative output factors for a typical commissioning dataset. The Monte Carlo calculated backscatter factors to the beam monitor chamber agree well with measured data in the literature. Monte Carlo simulations have also been shown to be able to accurately predict the collimator exchange effect and its component for rectangular fields. The information obtained is also useful to develop an algorithm for accurate beam modelling. This investigation indicates that Monte Carlo methods can be used to assist commissioning of output factors for photon beams.

Monte Carlo modelling of external radiotherapy photon beams

An essential requirement for successful radiation therapy is that the discrepancies between dose distributions calculated at the treatment planning stage and those delivered to the patient are minimized. An important component in the treatment planning process is the accurate calculation of dose distributions. The most accurate way to do this is by Monte Carlo calculation of particle transport, first in the geometry of the external or internal source followed by tracking the transport and energy deposition in the tissues of interest. Additionally, Monte Carlo simulations allow one to investigate the influence of source components on beams of a particular type and their contaminant particles. Since the mid 1990s, there has been an enormous increase in Monte Carlo studies dealing specifically with the subject of the present review, i.e., external photon beam Monte Carlo calculations, aided by the advent of new codes and fast computers. The foundations for this work were laid from the late 1970s until the early 1990s. In this paper we will review the progress made in this field over the last 25 years. The review will be focused mainly on Monte Carlo modelling of linear accelerator treatment heads but sections will also be devoted to kilovoltage x-ray units and 60Co teletherapy sources.

A three-source model for the calculation of head scatter factors

Accurate determination of the head scatter factor Sc is an important issue, especially for intensity modulated radiation therapy, where the segmented fields are often very irregular and much less than the collimator jaw settings. In this work, we report an Sc calculation algorithm for symmetric, asymmetric, and irregular open fields shaped by the tertiary collimator (a multileaf collimator or blocks) at different source-to-chamber distance. The algorithm was based on a three-source model, in which the photon radiation to the point of calculation was treated as if it originated from three effective sources: one source for the primary photons from the target and two extra-focal photon sources for the scattered photons from the primary collimator and the flattening filter, respectively. The field mapping method proposed by Kim et al. [Phys. Med. Biol. 43, 1593-1604 (1998)] was extended to two extra-focal source planes and the scatter contributions were integrated over the projected areas (determined by the detector's eye view) in the three source planes considering the source intensity distributions. The algorithm was implemented using Microsoft Visual C/C++ in the MS Windows environment. The only input data required were head scatter factors for symmetric square fields, which are normally acquired during machine commissioning. A large number of different fields were used to evaluate the algorithm and the results were compared with measurements. We found that most of the calculated Sc's agreed with the measured values to within 0.4%. The algorithm can also be easily applied to deal with irregular fields shaped by a multileaf collimator that replaces the upper or lower collimator jaws.

Modeling the output ratio in air for megavoltage photon beams

The output ratio in air, OR, for a high-energy x-ray beam describes how the incident central axis photon fluence varies with collimator setting. For field sizes larger than 3 x 3 cm2, its variation is caused by the scatter of photons in structures in the accelerator head (primarily the flattening filter and the wedge, if one is used) and by the backscatter of radiation into the monitor ionization chamber. The objective of this study was to evaluate the use of an analytical function to parametrize OR for square collimator setting c: OR = (1 + a1.c).[1 + a2.erf(c/lambda)2].H0. For open beams, these parameters can be attributed to explicit physical meanings within the systematical uncertainty of the model: a1 accounts for backscatter into the monitor, a2 is the maximum scatter-to-primary ratio for head-scattered photons, and lambda represents the effective width of the "source" of head-scatter photons. H0 is a constant that sets OR = 1 for c = 10 cm. This formula also fits OR for wedge beams and a Co-60 unit, although the fitting parameters lose their physical interpretations. To calculate the output ratio for a rectangular field, cx x cy, an equivalent square can be used: c = (1 + k).cy x cx/(k.cx + cy), where k is a constant. The study included a number of different accelerators and a cobalt-60 unit. The fits for square fields agreed with measurements with a standard deviation (SD) of less than 0.5%. Using k = lx.(f - ly)/ly.(f - lx), where lx and ly are the source-to-collimator distances and f is the source-to-detector distance, measurements and calculations agreed within a SD of 0.7% for rectangular fields. Sufficient data for the three parameters are presented to suggest constraints that can be used for quality assurance of the measured output ratio in air.

Backscatter towards the monitor ion chamber in high-energy photon and electron beams: charge integration versus Monte Carlo simulation

In some linear accelerators, the charge collected by the monitor ion chamber is partly caused by backscattered particles from accelerator components downstream from the chamber. This influences the output of the accelerator and also has to be taken into account when output factors are derived from Monte Carlo simulations. In this work, the contribution of backscattered particles to the monitor ion chamber response of a Varian 2100C linac was determined for photon beams (6, 10 MV) and for electron beams (6, 12, 20 MeV). The experimental procedure consisted of charge integration from the target in a photon beam or from the monitor ion chamber in electron beams. The Monte Carlo code EGS4/BEAM was used to study the contribution of backscattered particles to the dose deposited in the monitor ion chamber. Both measurements and simulations showed a linear increase in backscatter fraction with decreasing field size for photon and electron beams. For 6 MV and 10 MV photon beams, a 2-3% increase in backscatter was obtained for a 0.5 x 0.5 cm2 field compared to a 40 x 40 cm2 field. The results for the 6 MV beam were slightly higher than for the 10 MV beam. For electron beams (6, 12, 20 MeV), an increase of similar magnitude was obtained from measurements and simulations for 6 MeV electrons. For higher energy electron beams a smaller increase in backscatter fraction was found. The problem is of less importance for electron beams since large variations of field size for a single electron energy usually do not occur.

Two-effective-source method for the calculation of in-air output at various source-to-detector distances in wedged fields

A simple algorithm was developed for calculation of the in-air output at various source-to-detector distances (SDDs) on the central axis for wedged fields. In the algorithm we dealt independently with two effective sources, one for head scatter and the other for wedge scatter. Varian 2100C with 18 and 8 MV photon beams was used to examine this algorithm. The effective source position for head scatter for wedged fields was assumed to be the same as that for open fields, and the effective source position for wedge scatter was assumed to be a certain distance upstream from the physical location of the wedge. The shift of the effective source for wedge scatter, w, was found to be independent of field size. Moreover, we observed no systematic dependency of w on wedge angle or beam energy. One value, w = 5.5 cm, provided less than 1% difference in in-air outputs through the whole experimental range, i.e., 6 x 6 to 20 x 20 cm2 field size (15 x 20 cm2 for 60 degrees wedge), 15 degrees-60 degrees wedge angle, 80-130 cm SDD, and both 18 and 8 MV photon beams. This algorithm can handle the case in which use of a tertiary collimator with an external wedge makes the field size for the determination of wedge scatter different from that for head scatter. In this case, without the two-effective-source method, the maximum of 4.7% and 2.6% difference can be given by the inverse square method and one-effective-source method in a 45 degrees wedged field with 18 MV. Differences can be larger for thicker wedges. Enhanced dynamic wedge (EDW) fields were also examined. It was found that no second effective source is required for EDW fields.

Calculation of head scatter factors at isocenter or at center of field for any arbitrary jaw setting

The purpose of this work is to calculate the head scatter factors for any arbitrary jaw setting by using two different semi-empirical methods. The head scatter factor at the center of field (COF) for any arbitrary jaw setting can be defined as H(COF)(X1,X2,Y1,Y2,r)=DairCOF(XI1,X2,Y1,Y2,r)/ [Dair(5,5,5,5,0)*OAR(r)], where X1, X2, Y1, and Y2 are the jaw positions; r is the distance between COF and isocenter (IC); OAR(r) is the Off-Axis-Ratio; DairCOF(X1,X2,Y1,Y2,r) is the dose in air measured at COF; Dair(5,5,5,5,0) is the dose in air measured at IC for the 10 x 10 cm2 field. In certain clinical situations, doses are prescribed at IC instead of COF for asymmetric fields. In these cases, head scatter factors should be determined at IC. It is found that the head scatter factors at IC for asymmetric fields [H(IC)(X1,X2,Y1,Y2)] are lower than H(COF)(X1,X2,Y1,Y2,r) for the same jaw setting by up to 4%. The values of H(IC)(X1,X2,Y1,Y2) and H(COF)(X1,X2,Y1,Y2,r) for a variety of jaw settings were measured using a miniphantom of 3-cm diameter for a 6- and a 18-MV photon beams. An equivalent square formula, derived presently at the source plane for any jaw setting, was used to calculate H(COF)(X1,X2,Y1,Y2,r). The calculation and the measurement agree within +/-1% (+/-0.5% for most clinical situations). To calculate H(IC)(X1,X2,Y1,Y2), we have generalized the Day's "quarter-field" method, i.e., H(IC)(X1,X2,Y1,Y2) = [H(X1,X1,Y1,Y1) + H(X1,X1,Y2,Y2) + H(X2,X2,Y1,Y1) + H(X2,X2,Y2,Y2)]/4. We found that the calculation and the measurement agree within +/-0.8% for the beams studied.

A generalized solution for the calculation of in-air output factors in irregular fields

Three major contributors of scatter radiation to the in-air output of a medical linear accelerator are the flattening filter, wedge, and tertiary collimator. These were considered separately in the development of an algorithm to be used to set up an in-air output factor calculation formalism for open and wedge fields of irregular shape. A detector's eye view (DEV) field defined at the source plane was used to account for the effects of collimator exchange and the partial blockage of the flattening filter by the tertiary collimator in the determination of head scatter. An irregular field determined at the source plane by a DEV was segmented and mapped back into the detector plane by a field-mapping method. Field mapping was performed by using a geometric conversion factor and equivalent field relationships for head scatter. The scatter contribution of each segmented equivalent field at the detector plane was summed by Clarkson integration. The same methodology was applied for determining both tertiary collimator and wedge scatter contribution. However, the field size that determined the amount of scatter contribution was not the same for each component. For tertiary collimator scatter and external wedge scatter, a field projected to the detector plane was used directly. Comparisons of calculated and measured values for in-air output factors showed good agreement for both open and external wedge fields. This algorithm can also be used for multileaf collimator (MLC) fields irrespective of the position of the MLC (i.e., whether the MLC replaces one secondary collimator or is used as a tertiary collimator). The measurement and parameterization of tertiary collimator scatter is necessary to account for its contribution to the in-air output. Because a source-plane field is mapped into the detector plane, no additional dosimetric data acquisition is necessary for the calculation of head scatter.

The equivalent square concept for the head scatter factor based on scatter from flattening filter

The equivalent field relationship between square and circular fields for the head scatter factor was evaluated at the source plane. The method was based on integrating the head scatter parameter for projected shaped fields in the source plane and finding a field that produced the same ratio of head scatter to primary dose on the central axis. A value of sigma/R approximately equal to 0.9 was obtained, where sigma was one-half of the side length of the equivalent square and R was the radius of the circular field. The assumptions were that the equivalent field relationship for head scatter depends primarily on the characteristics of scatter from the flattening filter, and that the differential scatter-to-primary ratio of scatter from the flattening filter decreases linearly with the radius, within the physical radius of the flattening filter. Lam and co-workers showed empirically that the area-to-perimeter ratio formula, when applied to an equivalent square formula at the flattening filter plane, gave an accurate prediction of the head scatter factor. We have analytically investigated the validity of the area-to-perimeter ratio formula. Our results support the fact that the area-to-perimeter ratio formula can also be used as the equivalent field formula for head scatter at the source plane. The equivalent field relationships for wedge and tertiary collimator scatter were also evaluated.

Measurement of backscatter to the monitor chamber of medical accelerators using target charge

A simple noninvasive method is described for determining the backscatter to a monitor chamber of a medical accelerator based on the measurement of charge deposited in the target. This method is compared quantitatively to the more elaborate telescopic method for photon beams of 6 MV and 15 MV on linear accelerators having mica and Kapton monitor chambers. The new target charge method gives results consistent with the telescopic method to within 0.3%.

Monitor chamber backscatter for intensity modulated radiation therapy using multileaf collimators

Backscattered radiation into the machine monitor chamber can affect the machine output variation, with changes in field size and shape. For intensity modulated radiation therapy (IMRT) where many field, which may have small dimensions, are summed to give an intensity modulated field, the magnitude of backscatter will be different due to both the backscattering surface area changing, and the delivered monitor units being larger than for the equivalent static field. The effect of backscatter variation with field size for a Philips SL15 accelerator has been investigated at 8 MV for static and IMRT fields both in the standard clinical operating condition where an anti-backscatter plate is fitted, and also for a case where the anti-backscatter plate has been removed. The results show that in the absence of the anti-backscatter plate the variation in output between a 4 cm by 4 cm field and a 40 cm by 40 cm field size due to backscattered radiation was 5% for static fields. The anti-backscatter plate reduced this variation to less than 1%. When the accelerator operated in IMRT mode, with the backscatter plate in place, changes in the output due to additional backscattered radiation were less than 0.3%. With the backscatter plate removed, the outputs were lower, indicating the presence of additional backscattered radiation. It can be concluded that for the Philips MLC and SL accelerator with its anti-backscatter plate, the effects of backscattered radiation can be ignored for both static and IMRT fields.

Clinical implementation of a two-component x-ray source model for calculation of head-scatter factors

A calculation method is described, which presumes that linac output has two components. One component is direct x rays from the target and the remaining component is an extra-focal, distributed source of radiation that is scattered from the flattening filter, primary collimator, and the jaws of the moveable collimator. This calculation method gives values for output factors that differ from measured values by no more than 0.5% for field widths from 4 to 40 cm, rectangular fields with long to short axis ratios as great as 10, symmetric and asymmetric fields, and source-to-axis distances from 65 to 360 cm. While this calculation method has great accuracy and flexibility, a minimal amount of input data is required: (1) measured output factors at a source-to-axis distance of 100 cm for square fields; (2) positions of the collimator with respect to the x-ray source target; and (3) output factors measured at various source-to-axis distances for a 10 cm x 10 cm field.

Equivalent square as a predictor of depth of dose maximum for megavoltage therapy beams

The depths of dose maxima, dmax, and surface doses of 6 MV, 10 MV, and 18 MV photon beams were measured for various square fields and rectangular fields with elongation ratios from 1 to 27. Rectangular fields with elongation ratios below 2 have essentially the same depths of dose maxima and surface doses as their corresponding equivalent square fields. For rectangular fields with elongation ratios above 2, the surface doses increase, and depths of dose maxima decrease with increasing elongation ratio in comparison to their respective values for their corresponding equivalent square fields. The shift of dmax toward the surface is more pronounced when the upper jaws rather than the lower jaws define the long axis of the field. This collimator exchange effect does not influence the surface dose. Even for the largest elongation ratios, the changes in dmax and surface dose from their equivalent square field values were minor and clinically insignificant, suggesting that the equivalent square approach provides a reliable method for predicting the values of dmax and surface dose for rectangular fields from square fields data.

Measurement of photon beam backscatter from collimators to the beam monitor chamber using target-current-pulse-counting and telescope techniques

Backscattered radiation (BSR) arising from field-defining collimators and entering the beam monitor chamber (BMC) may contribute to observed variations in medical linear accelerator photon beam output with collimator setting. Measuring the magnitude of such contributions for particular accelerators under specified operating conditions is therefore important when attempting to understand and model accelerator head scatter. The present work was conducted to confirm some backscatter measurements for collimating jaws reported previously and to extend these to include other accelerators and a multileaf collimator (MLC). BSR reaching the BMC from the jaws of Clinac 600C, 2100C and 2300CD accelerators and from an MLC on the 2300CD was investigated using both target-current-pulse-counting and telescope methods. Our measurements show that for the Clinac 600C BSR-dependent output variations are negligible. However, for the 2100C and 2300CD BSR-dependent relative output increased in an almost linear fashion, by up to 2.4% for 15 and 18 MV beams, and by up to 1.7% for 6 MV beams, as the field size varied from 5 x 5 cm2 to 40 x 40 cm2. The magnitude of BSR dependent upon collimator location in the head, as expected, thereby contributing to the collimator exchange effect. An earlier study at our centre using the telescope method had reported higher BSR levels. This discrepancy was resolved when corrections for telescope block and room scatter, previously assumed negligible, were made.

Characteristics of contamination electrons in high energy photon beams

A simple method to estimate the contribution of contaminating electrons to the dose, and to evaluate their dosimetric characteristics is proposed. The method is based on a normalisation of the tissue--maximum ratio curves to a constant primary photon fluence. The contribution of the contaminating electrons to the dose is calculated by subtracting the dose relative to a small field from the dose relative to the field under consideration. The method includes the determination of the mean energy, the linear apparent attenuation coefficient, the 50% range and the maximum range of the contaminating electrons. The extrapolated surface dose normalised to a constant primary photon fluence has been found to be constant for a constant collimator opening whatever may be the source distance.

Prediction of Saturne II+ 10 MV and 23 MV photon beam output factors

A two-variable, three-parameter formula for modified, virtual equivalent square sides has been worked out which, when incorporated into empirical output-square field size function, generates output values of rectangular fields taking into account the asymmetry of the collimating system of the Saturne II+ accelerator generating 10 MV and 23 MV photon beams.

The determination of phantom and collimator scatter components of the output of megavoltage photon beams: measurement of the collimator scatter part with a beam-coaxial narrow cylindrical phantom

The separation of the total scatter correction factor Sc,p in a collimator scatter component, Sc, and a phantom scatter component, Sp, has proven to be an useful concept in megavoltage photon beam dose calculations in situations which differ from the standard treatment geometry. A clinically applicable method to determine Sc is described. Measurements are carried out with an ionization chamber, placed at a depth beyond the range of contaminant electrons, in a narrow cylindrical polystyrene phantom with a diameter of 4 cm of which the axis coincides with the beam axis. Sc,p is measured in a full-scatter phantom and Sp can be derived from Sc,p and Sc. In order to obtain a reliable separation, i.e. excluding the influence of contaminant electrons, measurements of Sc,p have been carried out at depths of 5 cm for photon beams with a quality index (QI) up to and including 0.75 and a depth of 10 cm with QI larger than 0.75. These depths are in accordance with recommendations given in recent dosimetry protocols. The consistency of the method was checked by comparing calculated and measured values of Sc,p for a set of blocked fields for a range of photon beam energies from 60Co up to 25 MV showing a maximum deviation of 2%. The method can easily be implemented in existing procedures for the calculation of the number of monitor units to deliver a specified dose to a target volume.

The quality of high-energy X-ray beams

Supplement 17 of the British Journal of Radiology is a survey of central-axis depth doses for radiotherapy machines, patterned largely on BJR Supplement 11 published some 11 years earlier. Inspection of the high-energy X-ray depth doses for a 10 x 10 cm field at an SSD of 100 cm disclosed large differences between the two sets of data, especially for qualities above 8 MV. For example, a depth dose of 80% at 10 cm is rated at about 19 MV according to BJR Supplement 11, and 23 MV according to BJR Supplement 17. It was found that the Supplement 17 depth-dose data above 8 MV were erratic, but that the Supplement 11 data could be represented by an analytical expression, providing a unique means of assigning MV quality. It was also found that the dose-weighted average energy of the filtered beam plotted smoothly against depth dose. For dosimetric purposes, it is suggested that this parameter be used as a true measure of beam quality, removing the discrepancies introduced by the use of nominal MV for this purpose.

Methods of calculating the output factors of rectangular electron fields

The dose output of a clinical electron beam exhibits a complex dependence upon field size, beam energy and collimation system design. A variety of methods have been developed in the past to calculate the output of an electron beam of arbitrary field dimensions. This paper describes three of these methods and indicates the advantages and disadvantages of each. Comparisons with measured data are also presented.

Dosimetric characteristics of a 6 MV photon beam from a linear accelerator with asymmetric collimator jaws

Dosimetric measurements have been made of a 6 MV photon beam from a linear accelerator equipped with asymmetric jaws. The field size factors for asymmetrically set fields are compared to those for symmetrically set fields. The change of beam quality has been measured as a function of off-axis position of the asymmetric fields to assess its effect on depth dose. Additional measurements include beam penumbra and shape of isodose curves for open and wedge fields as the field opening is moved asymmetrically from the central ray.

Effect of collimator setting on the output of rectangular fields from linear accelerators

The output (cGy/mu) of a rectangular field from a linear accelerator is not always the same as that of its equivalent square field. We have summarized output variations with upper and lower collimator setting for 4, 6, and 24 MV X-rays. It is concluded that an error in output on the order of a few percent is introduced for elongated fields if lower set of collimator jaws is used for setting the longer dimension of the field, and computing the output using equivalent square method. It is recommended that specific guidelines be developed regarding rectangular field setting on high energy linacs.

Spinal cord protection during radiation therapy

Treating intrathoracic malignancies to high doses, particularly those of lung and esophagus, requires limiting the radiation dose delivered to the spinal cord. Several factors are important in determining the cord dose. These are: The distance from the block or collimator edge to the cord, the variation of dose with distance from the block or collimator edge and, the expected variation of this distance for clinical set-up from day-to-day. When treating with an oblique beam, the position of the cord may be difficult to identify. A technique for localizing the spinal cord on a simulator film at an arbitrary gantry angle is presented. The technique requires determination of distances from the central axis of the beam to the medial aspect of the pedicle and posterior vertebral body. These can readily be obtained from measurements on orthogonal, AP/PA and lateral isocentric simulator radiographs. A mathematical transformation is applied to determine the corresponding cord locations on the oblique radiographs for any arbitrary gantry angle. The accuracy of cord localization was within 2-3 mm with a precision of 2 mm for five physicians who used this technique. The beam edge characteristics for 60Co, 6 MV, and 10 MV teletherapy unit were measured for various depths and field sizes. For the 6 and 10 MV units, the beam penumbra is nearly independent of the field size, depth and field defining devices (inner and outer collimator jaws, trimmer bars, and shielding blocks). Because the beam penumbra is dependent on the design of the linear accelerator, its measurement should be made individually for each linear accelerator. Our preliminary data on patient positioning uncertainty did not exceed the 6-8 mm limit documented in the literature.

On the use of a quality index to specify high energy photon beams

It is important to specify the beam quality in a simple and nonambiguous way in order on one hand to make comparisons easier between treatments performed in various hospitals, or at different times in the same hospital and on the other hand to facilitate the choice of numerical values for factors like restricted mass-collision stopping-power ratios and perturbation correction factors used in the conversion of ionization measurements into absorbed dose. We have adopted for high-energy photon beam specification a quality index (I) defined by the ratio (I20/I10) of ionizations measured with a constant source-detector distance for a reference field size 10 X 10 cm2. We have found that this quality index is independent of the source detector distance. On the other hand, the apparent linear attenuation coefficient measured on the exponential part of the tissue-maximum ratio curve can be calculated for any field size from the value of I for most high energy photon beams. In order to check the validity of the quality index for other linacs from other manufacturers, we have compared our results to published data related to various photon beams in a wide energy range: 2.5 to 45 MV.